The objective is to compare transcriptionally active and inactive chromatin with respect to (a) initial yields of ionizing radiation-induced DNA damage and (b) repair capacity. Specific model systems will be ribosomal chromatin, which contains the DNA coding for ribosomal RNA (rRNA) and chromatin containing genes active in mRNA synthesis. Comparison will be made between the active DNAs of proliferating cells and the less active DNAs of plateau phase (generally more radio-resistant) cells. Rodent and human cell lines will be obtained during exponential growth, during plateau phases induced by nutrient or serum depletion, confluency, or drugs, or during stimulation with nutrients, serum, or insulin. The rates of rRNA and poly(A plus) RNA syntheses and radiation damage to and repair of DNA will be determined. We have developed a system to analyze single-strand breaks, alkali-labile lesions, and enzyme-sensitive sites in specific DNAs. The procedure involves releasing the genes from bulk nuclear DNA with a restriction endonuclease, treatment with a lesion-specific endonuclease, and sizing the DNA fragments by agarose gel electrophoresis, then identifying the DNAs by hybridization to specific probes. Unlike the majority of nuclear DNA, the rRNA genes and some others are known to be differentially active in proliferating and plateau phase cells. Plateau phase cells have been considered a suitable two-dimensional model for the non- or slowly-growing and generally more radioresistant sub-populations of tumors. Thus, the results will help to define some critical areas of metabolism affected when nutrients or hormones simulate the proliferation of plateau phase cells and enhance their radiosensitivity.